In recent years, in the field of information recording, research concerning optical information recording systems has been going on in many places. The optical information recording system has many advantages such as the ability to record and/or reproduce information without contact and the ability to handle read only type, writable type, and rewritable type memory formats. Broad applications from industrial use to consumer use are conceivable for this as a system enabling realization of inexpensive large sized files.
In the optical pickup mounted in a recording and/or reproduction apparatus of a CD (compact disk), DVD (digital versatile disk), or other optical recording medium (hereinafter also referred to as an “optical disk”) for the above various types of optical information recording systems (hereinafter also referred to as an “optical disk drive”), laser light of a wavelength of for example 780 nm or 650 nm is emitted from a laser diode and focused on an optical recording layer of the optical disk by an optical system including a beam splitter and other optical members. Light reflected from the optical disk follows a reverse path in the above optical system and is projected onto a photodiode or other light receiving element by a multi lens or the like.
Information recorded on the optical recording layer of the optical disk is read from the changes in the light reflected from the optical disk.
The optical disk drive described above has, in the usual configuration, a light source for emitting light of a wavelength λ, an optical system including an object lens (condensing lens) having a numerical aperture NA for focusing the light emitted by the light source on the optical recording layer of a optical recording medium, a light receiving element for detecting the light reflected from the optical recording layer, and the like.
In the above optical disk drive, a spot size φ of the light on the optical recording layer is generally given by the following equation (1):φ=λ/NA  (1)
The spot size φ of the light has a direct influence upon the recording density of the optical recording medium. The smaller the spot size φ, the higher the recording density possible and the larger the capacity.
Namely, this shows that the shorter the wavelength λ of the light or the larger the numerical aperture NA of the object lens, the smaller the spot size φ, so a higher recording density is possible.
According to the above indicator, in order to realize a larger capacity of an optical disk, an optical disk drive wherein for example the wavelength of the light source is shortened from green to blue and further to an ultraviolet ray region and the numerical aperture NA of the object lens is raised to for example about 0.8 to 2.5 is being investigated.
When the numerical aperture of the object lens becomes larger as described above, in general, the allowable disk tilt in the optical disk drive is reduced. Therefore, in order to cope with this, it becomes necessary to obtain an optical disk of a type wherein the thickness of a protective layer on a light incident side of the optical disk is made thinner down to about 1 μm to 0.1 mm.
An object lens having a numerical aperture NA raised to about 0.85 can be realized by for example a solid immersion lens (hereinafter also referred to as an “SIL”)—one type of combination lens.
FIG. 1 is a sectional view of the schematic configuration of an SIL. The SIL is comprised by a first convex lens L1 and a second convex lens L2.
Further, FIG. 2A is a plan view of the first convex lens, and FIG. 2B is a sectional view along A-A′ in FIG. 2A. Further, FIG. 2C is a sectional view of the second convex lens.
The first convex lens L1 is comprised of a lens body 1 provided with convex aspherical surfaces at both surfaces (1c, 1d), while the second convex lens L2 is comprised of a lens body 2 provided with a convex spherical surface at one surface 2c and with a flat surface FL at the other surface 2d. 
The first convex lens L1 and the second convex lens L2 are arranged on an identical optical axis AX and comprised so that a laser beam LB from a light source LS passes through the first convex lens L1, then passes through the second convex lens L2 and is focused at a predetermined point on the optical axis AX at a side opposite to the first convex lens L1.
In order to make the aberration of the first convex lens L1 smaller, it is necessary to bring the centers of the convex aspherical surfaces provided at both surfaces (1c, 1d) into register with a high precision.
In order to realize this, a mold shown in FIG. 3 is used to form the first convex lens L1.
FIG. 3A is a plan view of the mold, while FIG. 3B is a sectional view along A-A′ in FIG. 3A.
The mold is comprised of a cylindrical first mold Ma into which are inserted from above and below a pin-shaped second mold Mb and third mold Mc having aspherical concave surfaces. A space surrounded by the inside wall surfaces of the first mold Ma, second mold Mb, and third mold Mc becomes a molding cavity Cav.
To use the above mold to form the first convex lens L1, as shown in FIG. 4A, ball glass BG is placed in the cavity Cav, the mold is heated up to a temperature where the glass softens, then, as shown in FIG. 4B, the second mold Mb and the third mold Mc are pressed from above and below to form the lens.
The above mold is structured with center axes of the cylindrical first mold Ma and pin-shaped second mold Mb and third mold Mc in register, so it is relatively easy to bring the centers of the convex aspherical surfaces provided at both surfaces of the first convex lens into register with a high precision during the processing.
Further, in order to make the aberration of the SIL comprised of the first convex lens L1 and the second convex lens L2 smaller, it is necessary to make the inclination of the optical axis of the first convex lens L1 and the second convex lens L2 as small as possible.
As shown in FIG. 2B, however, in the first convex lens L1, the convexities provided at the two surfaces (1c, 1d) are aspherical surfaces, so the surfaces cannot be used as reference surfaces for positioning.
Accordingly, in order to secure a reference surface for positioning, the first convex lens L1 has been provided with a flange 1e having a flat surface FL at the outer circumference of the lens body 1 and the optical axis adjusted using the surface of the flange 1e as a reference.
In recent years, however, the numerical aperture of lenses has become increasingly larger. At the same time, an extremely high precision is now being demanded in the adjustment of the optical axis.
For this reason, high precision adjustment satisfying the demands by only using a small area of flange portion provided at the outer circumference of the lens body as a reference surface as described above is becoming impossible.
As one method for solving this problem, there is the method of simultaneously forming a plurality of combination lenses. This will be explained below.
FIG. 5 is a sectional view of the schematic configuration of a combination lens (SIL) formed by the above method. The SIL is comprised of a first convex lens L1 and a second convex lens L2.
Further, FIG. 6A is the perspective view of the first convex lens, and FIG. 6B is a perspective view of the second convex lens.
The first convex lens L1 is comprised of the surface of a concavity 1a having a convex bottom provided at an upper surface of the lens body 1 and the surface of a concavity 1b provided at a lower surface of the lens body 1. These upper surface SD and a lower surface SA of the lens body 1 except for the concavities (1a, 1b) are flat surfaces able to serve as reference surfaces.
On the other hand, the second convex lens L2 is comprised of the surface of a concavity 2a provided at the upper surface of the lens body 2 and the lower surface of the lens body 2. These upper surface SB and lower surface SC of the lens body 2 except for the concavity 2a are flat surfaces able to serve as reference surfaces.
The first convex lens L1 and the second convex lens L2 are arranged on the identical optical axis AX. The lower surface SA of the lens body 1 and the upper surface SB of the lens body 2 are fixed by bonding. They are configured so that a laser beam LB from the light source LS passes through the first convex lens L1, then passes through the second convex lens L2 and is focused at a predetermined point on the optical axis AX at a side opposite to the first convex lens L1.
The method of production of the above SIL will be explained below.
First, the mold shown in FIG. 7A is used to form a first lens aggregate 10 comprised of a plurality of the above first convex lenses joined together. FIG. 7B is a sectional view of the first lens aggregate 10 formed in this way, and FIG. 7C is a plan view. The section along A-A′ in FIG. 7C corresponds to FIG. 7B.
The above mold has a first mold M1 and a second mold M2. Through holes are formed at predetermined positions. Pin molds P1b having convex surfaces at their front ends are inserted into the first mold M1, while pin molds P1a having concave surfaces at their front ends are inserted into the second mold M2. Further, positioning mark pins P11 are inserted in the second mold M2.
A space surrounded by the inside wall surfaces of the first mold M1, second mold M2, pin molds P1a, pin molds P1b, and positioning mark pins P11 becomes the molding cavity Cav.
By filling the interior of the mold having the above configuration by softened glass or another optical material, a first lens aggregate 10 comprised of a plurality of (nine in the figure) first convex lenses formed with concavities 1a having convex bottoms and concavities 1b forming first convex lenses L1 joined together and provided with positioning marks 11 as shown in FIG. 7B and FIG. 7C is formed.
On the other hand, a mold and method similar to those described above are used to form a second lens aggregate 20 comprised of a plurality of (nine in the figure) second convex lenses formed with concavities 2a forming the second convex lens L2 joined together and provided with positioning marks 21.
The first lens aggregate 10 and the second lens aggregate 20 obtained as described above are adhered together by superimposing the positioning marks (11, 21) and using an adhesive or the like.
In steps after this, the aggregate is divided into individual SILs having predetermined sizes as shown in FIG. 5 by predetermined division lines.
According to the above method of production of an SIL, when adhering together the first lens aggregate 10 and the second lens aggregate 20, the positioning can be carried out using the large area bottom surface of the first lens aggregate 10 and top surface of the second lens aggregate 20 as the reference surfaces, so it is possible to easily form the SIL with an extremely high accuracy while eliminating inclination of the optical axes of the first convex lens L1 and the second convex lens L2 without adjustment requiring a high level of skill.
However, in the mold for forming the first lens aggregate forming first convex lenses L1, through holes are provided in the first mold M1 and the second mold M2 at the positions forming the first convex lenses and pin molds P1b and pin molds P1a are inserted into them. In order to form the first convex lenses L1 with a high precision, it is necessary to bring the center axes of the pin molds P1b and the pin molds P1a into register. As shown in FIG. 7, however, in a general mold structure, the first mold M1 and the second mold M2 having the through holes for insertion of the pin molds P1b and the pin molds P1a are formed separately and then combined, so it is very difficult to match the center axes of the pin molds P1b and the pin molds P1a and therefore it has become difficult to obtain high precisely formed first convex lenses.